1. Field of the Invention
The invention relates primarily to the field of A.C. arc welding and, more particularly, to an apparatus and method for deriving from an A.C. source a square-wave current whose voltage at each polarity reversal is high enough to insure conduction when the work piece is aluminum, and whose positive and negative half-cycles may be individually controlled in both height and duration.
2. Description of the Prior Art
In the prior art one common solution to overcoming the problem of rectification and tungsten migration in the arc welding of aluminum has been to employ a balanced wave supply as shown, for example, in U.S. Pat. No. 2,472,323. The balanced wave supply consists of a capacitor in series with an A.C. supply. Most of the source voltage drops appears across the capacitor, and no D.C. component due to rectification can develop because the capacitor will conduct only A.C. In addition, a voltage, equal to the peak of the supply voltage plus the voltage due to the charge on the capacitor remaining after a forward half-cycle of conduction, is available to force reverse conduction when electrons are poorly emitted by the aluminum work piece. However, such a balanced wave supply is very large and expensive due to the need for a variable A.C. voltage and a large bank of capacitors.
The less expensive power supplies for gas tungsten arc welding (GTAW) or TIG welding of aluminum utilize magnetic amplifiers or saturable reactors which provide control of the conduction angle on each half-cycle. However, the conduction angle becomes unbalanced on alternate half-cycles due to the larger voltage drop when the aluminum must emit, thereby resulting in unequal reset of the reactor cores. Furthermore, the wave shape of the welding current in this type of phase-control power supply is more peaked than a sine wave and can result in both tungsten migration due to the intensity of electron emission by the tungsten on the forward half-cycle and also overheating and excessive melting of the tungsten during the reverse half-cycle when the tungsten absorbs bombarding electrons.
Back-to-back thyristors or a triac can provide either balanced or selectively unbalanced conduction on the forward and reverse half-cycle, but again the resulting large current peaking is worse than a sine wave. Furthermore, if an inductor is used in the A.C. line to reduce peaking, it will have the correct inductance for only one current level, whereas a wide range of current levels must be accommodated in actual practice.
In addition, resistance has been added in series with such phasecontrol supplies in an attempt to obtain a depeaking effect proportional to the welding current, but this technique is only partially successful and requires a larger supply due to poor efficiency.
Another attempted solution of the prior art has been to add a diode bridge in series with the A.C. line with an inductor connected across the D.C. terminals of the bridge. Such an arrangement is shown in U.S. Pat. No. 3,364,334, for example. However, in such an arrangement, the bridge is not the control element, and the welding current control must still be provided on the A.C. lines. Such a diode bridge is also incapable of producing an asymmetrical square-wave (i.e., one in which the adjacent positive and negative half-cycles are of different widths) which is highly desirable for preventing tungsten migration when welding aluminum. Such a circuit also suffers severe inadequacies due to the fact that the diodes form a freewheeling path at all times. This prior art circuit will be described in more detail below.